Secure communications systems are known. The goal of such systems is to render a message unintelligible so as to prevent unauthorized reception. Typically, a message, such as a voice message, is digitized and processed through an encryption device to produce a signal that is random or pseudo-random in nature, thus appearing noise-like to the unauthorized receiver. The particular encryption algorithm used by the encryption device may be a proprietary algorithm, or may be based on a standard such as the Data Encryption Standard (DES) promulgated by the National Bureau of Standards (NBS), of the United States Government.
Generally, it is known to operate a selected encryption device in the cipher feedback (CFB) mode, wherein the cipher text output signal is added (modulo two) with the plain text input signal; the result being feedback into the encryption device. As used herein, cipher text means the encrypted unintelligible signal transmitted from, or received by, an encryption device operating within a secure communication system. Further, as used herein, plain text means the unencrypted (or "clear") signal transmitted from, or received by, an encryption device operating within a secure communication system. Those skilled in the art will appreciate that the wide utilization of the CFB mode of operation was due chiefly to the self-synchronizing signal (as related to encryption synchronization) that is produced. In fact, the NBS has suggested that secure communication equipment suppliers employing the D.E.S. design their systems to operate in the CFB mode.
However, a secure communication system having transceiving devices that operate in the CFB mode suffers a serious detriment in that the transceiving devices have a reduced operational range. The range reduction is caused by error multiplication in the receiver, which manifests itself as degraded receiver sensitivity. The error multiplication is fundamental to the CFB concept since an erroneously received bit is applied to the encryption algorithm input until the error finally "clears" or is shifted out of the algorithm input register. Thus, for D.E.S. with an input register of 64 bits, a single erroneously received bit produces sixty-four errors in the plain text output stream. Receiver sensitivity reduction of 4-6 dB is typical, and, depending on the transmitter power, may result in a range reduction measured in miles. Therefore, a need exists for a secure communication system to overcome the range reduction problem of the prior art.